1. Field of the Invention
The present invention relates to an information encoding apparatus and method, an information decoding apparatus and method, a recording medium, and a program and more particularly, to an information encoding apparatus and method, an information decoding apparatus and method, a recording medium, and a program that allow data to be embedded in audio signals.
2. Description of the Related Art
Hitherto, as a method for compressing audio signals, there has been known a method in which frequency components not reaching a minimum audible level curve are removed. The minimum audible level curve is collective experimental values of the human-audible levels of pulse sounds having various different frequencies in a quiet environment. The curve is denoted as L in FIG. 1.
Referring to FIG. 1, the axis of abscissa indicates the frequencies of the spectra of audio signals, while the axis of ordinate indicates the levels of the spectra of audio signals. The level of spectrum A is greater than the value in the minimum audible level curve at that frequency, meaning that the level of spectrum A is above the minimum level audible to human ears, so that people can hear it. The level of spectrum B is below the value in the minimum audible level curve at that frequency, meaning that the level of spectrum B is below the minimum level audible to human ears, so that people cannot hear it.
Conventionally, compression of audio signals has been accomplished by removing the spectra whose levels are below the values in the minimum audible level curve at particular frequencies (inaudible), as indicated by spectrum B in FIG. 1. This compression method is used also in the moving picture experts group (MPEG) and other types of audio encoding methods.
Conventional compression methods, however, have been disadvantageous in that they cannot allow information to be embedded while compressing audio signals at the same time. For example, it is possible to directly embed information in a spectrum having a level below a value in the minimum audible level curve at the frequency. The embedded information, however, will add to the amount of information, because the spectrum having the embedded information cannot be cut off.